The methanol cut, produced either by a cracking or steam reforming processes, can produce gas mixtures of markedly different compositions. Both the suitable choice of catalysts and the operating conditions determine in great part the composition of the resulting gas flow.
The use of methanol as a source in production of hydrogen, although very well known, has been considered to be rather disadvantageous, especially with regard to economics. However, the decreasing availability of hydrocarbons, as well as gasification of coal, tend to change this situation.
Existing techniques have already been improved and adapted to achieve an optimal use of methanol as the initial hydrocarbon for production of gas with varied compositions.
The technique of steam reforming seems particularly well suited to the production of hydrogen, a part of the introduced water being transformed into hydrogen; and under conditions of low-temperature use it is advisable to resort to a catalyst not having any activity in regard to methanization.
A process is known from French patent No. 2,490,615 for reforming methanol to obtain gas containing variable concentrations of hydrogen and carbon monoxide, the balance of the gas being made up of carbon dioxide, methane, steam and optionally nitrogen, according to to which an operation is performed of cracking mixtures in variable portions of steam and methanol vapor, in the presence optionally of nitrogen or air and gas which are recycled, after optional separation of certain components and gas coming from outside the unit. This process used in several stages makes it possible to produce pure hydrogen.
In stage a) heating and vaporization is performed at the required pressure, for example 20 bars, of the adjusted methanol- water feed mixture.
Then in stage b), reforming, on suitable catalyst, of the methanol-water mixture is performed at a temperature of about 300.degree. C.
This operation is performed according to two simultaneous reactions in equilibrium, which together constitute the reforming:
Decomposition of methanol: EQU CH.sub.3 OH.revreaction.CO+2H.sub.2
Conversion of carbon monoxide: EQU CO+H.sub.2 O.revreaction.CO.sub.2 +H.sub.2
These two reactions can be performed either on a single catalyst or on two catalysts.
The gas obtained at the output of the reforming process, at 300.degree. C., has the following approximate composition, expressed in molar percentage, for a ratio CH.sub.3 OH/H.sub.2 O=0.5 at the input of the reactor: hydrogen (H.sub.2)=about 55.2%, carbon dioxide (CO.sub.2)= about 17.8%, carbon monoxide (CO)=about 0.9%, water (H.sub.2 O)=about 26.1%, with traces of methane or methanol.
In stage c) the reformed gas is cooled to ambient temperature with condensation of water.
After cooling, under 20 bars for example, the composition of the mixture is the following: H.sub.2 =about 74.3%, CO.sub.2 =about 24.0%, CO=about 1.2%, H.sub.2 O=about 0.5%, with trace of methane and methanol.
Then in last stage d), purification of the gas mixture is performed on a molecular sieve. This process is advantageous in combination with steam reforming of methanol, particularly for production of very pure hydrogen.
Purification of the gas mixture under said pressure by adsorption of gases other than hydrogen in a unit of the "Pressure Swing Adsorption," PSA, type unit, makes it possible, on the one hand, to obtain a very pure hydrogen fraction under pressure, constituting the product of the reforming unit and, on the other hand, a so-called "residual gas" fraction available at reduced pressure and whose typical composition expressed by volume is the following: H.sub.2 =about 47%, CO.sub.2 =about 48.2%, CO=about 3.4%, H.sub.2 O=about 1.2%, CH.sub.4 =about 0.2%.
According to the process treating practically pure methanol, the water condensed during phase c) is recycled at the input of the unit. The heat necessary for the reforming is generally obtained by heating with a heat-exchanging fluid at about 320.degree. C., in a combustion zone, where the residual gas fraction is burned. This heat-exchanging fluid is then the object of a heat exchange with the catalytic reforming zone, the catalyst tubes, on the one hand,--isothermal type reforming--and with the gas mixture to be reformed, on the other hand, particularly to assure its vaporization.
The above described process leads to very good results from practically pure methanol. In case the methanol contains a certain number of impurities consisting of alcohol heavier than methanol, and if the usual operating conditions of the catalyst are maintained, i.e., at a temperature of about 300.degree. C., the impurities of the methanol react only very little on the catalyst, on the one hand, and reduce the yield of reforming of the methanol, on the other hand. Consequently, there is an enrichment of the condensed water, in phase c), with methanol and various alcohols. This enrichment is in relation to the water/methanol ratio of the gas mixture entering on the catalyst. The following table gives some experimental values.
______________________________________ Composition of condensed liquid H.sub.2 O/CH.sub.3 OH (% by weight) 1.75 1.5 ______________________________________ Water 80.5 73.45 Methanol 15.6 22.00 Ethanol 1.15 1.35 Isopropanol 0.80 0.90 Propanol 1.05 1.20 Butanol 0.90 1.10 ______________________________________
An improved reforming process has been sought making it possible to treat methanol containing impurities consisting mainly of ethanol, isopropanol, propanol, butanol, various fatty alcohols, etc. . . . while reducing the drawbacks inherent in the process described above.